Multisite: Only query for one site in domain_exists().

`get_sites()` queries for a maximum of 100 records by default. In `domain_exists()`, we only use one.

A previous commit, [41736], has this same commit message but applies to `get_blog_id_from_url()` and #42073 instead.

Props danieltj, spacedmonkey.
Fixes #42072.

Built from https://develop.svn.wordpress.org/trunk@41738


git-svn-id: http://core.svn.wordpress.org/trunk@41572 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/ms-functions.php

@@ -1362,6 +1362,7 @@ function domain_exists( $domain, $path, $network_id = 1 ) {
 		'domain'     => $domain,
 		'path'       => $path,
 		'fields'     => 'ids',
+		'number'     => 1,
 	);
 	$result = get_sites( $args );
 	$result = array_shift( $result );

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9-alpha-41737';
+$wp_version = '4.9-alpha-41738';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.